The present invention relates to a pigment, a process for producing the pigment and a paint using the pigment, and more particularly, relates to a pigment suitably used for not only organic solvent-type paints but also water-based paints and having excellent dispersion facility, dispersion homogeneity and dispersion stability, a process for producing the pigment, and a paint produced by using the pigment.
Hitherto, inorganic pigments have been used as coloring agents for paints by dispersing these pigments in vehicle, because they are excellent in chemical resistance, weather resistance or the like, and has a high stability.
In recent years, there is an increasing demand for high performance and high quality of coating films obtained by applying a paint on various substrates and drying the applied paint. In order to meet such a demand, there have been strongly required improvements in properties of the pigment which are considered the most influential factor on the qualities of the coating films such as brightness of color tone, tinting strength, brightness and gloss. Further, it has been required that the pigment used in the paint shows an excellent dispersion facility, dispersion homogeneity and dispersion stability in a vehicle for the paint.
This fact is pointed out, for instance, in "The Latest Technique for Pigment Dispersion", published by the Technical Information Society in 1993, on page 15: ". . . Pigment is never used singly and the pigment is always used after dispersed in a binder resin, solvent or other suitable material. The fine pigment particles need to be easily dispersible and also homogeneously dispersed in the binder for obtaining a clear color tone, high tinting strength and other desired performance. However, since the more the fine particles, the less the stability thereof becomes, it is a matter of great account to maintain stable dispersibility of the pigment particles. . . . ", and ". . . The properties required for the pigment may be defined and classified as follows from the various viewpoints. Dispersibility is classified as a matter that affects the physical properties, but the dispersibility also has close relation to and an influences the optical, chemical and specific functional properties. Namely, the improvement of dispersibility of the pigment for allowing uniform dispersion not only to mean enhancement of clearness of color tone, tinting strength, brightness and gloss of the coating films, but also to lead to improvement of chemical properties such as fastness, and workability. . . . "
Paints are divided into two types depending on the solvent used: organic solvent-type paints using an organic solvent as main solvent and water-based paints using water as main solvent.
In recent years, water-based paints have been considered favorable in terms of safety, hygiene and environmental protection, because the water-based paints have no serious atmospheric pollution problem, are more advantageous in saving of resource and energy, and also are easy to handle, thereby causing little risk of a fire.
However, since the dispersion mechanism of the pigment particles in a water-based paint is different from that in the conventional organic solvent-type paints, it is impossible to disperse the pigment particles in a vehicle by the same dispersion techniques as used for the organic solvent-type paints. An aqueous resin (color developer) is used in a water-based paint. Although an organic solvent-type resin (color developer) exists in a dissolved state with a certain spread, the aqueous resin exists mostly in the form of particles such as emulsion polymer particles or colloidal particles, resulting in difficulty in exhibiting steric hindrance effect of the resin, and therefore, deteriorated dispersibility of the pigment particles in the resin. Consequently, it becomes a problem how the pigment particles should be dispersed in the vehicle and how long it should take until effected, that is, dispersion facility and dispersion homogeneity of pigment particles becomes the problem of primary consideration. Further, the dispersion stability comes into problem since the pigment particles tend to reagglomerate due to interaction between dispersed pigment particles and the aqueous resin particles.
Therefore, it has been strongly required to provide pigment particles which can sufficiently function for not only the organic solvent-type paints but also the water-based paints, and are excellent in dispersion facility, dispersion homogeneity and dispersion stability.
Conventionally, in order to improve dispersibility of pigment particles in vehicle, it has been proposed to coat pigment particle surfaces with various kinds of inorganic or organic compounds.
For example, in Japanese Patent Application Laid-open (KOKAI) No. 5-225552(1993), there are described ferromagnetic metal fine particles whose surfaces are treated with 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid) as a 4-membered ring compound, and a magnetic recording medium using the particles. In Japanese Patent Application Laid-open (KOKAI) No. 5-225553(1993), there are described ferromagnetic metal fine particles whose surfaces are treated with aromatic organic acids having an anthraquinone skeleton such as alizarin, and a magnetic recording medium using the particles.
Also, in Japanese Patent Application Laid-open (KOKAI) No. 8-120191(1995) which corresponds to U.S. Pat. No. 5,686,012, there are described colored inorganic particles whose surfaces are coated with squaric acid and then with polysiloxane modified with polyether or the like.
In addition, various methods have been attempted to modify surfaces of the pigment particles with alcohol or the like. For instance, there have been proposed a method of imparting a lipophilic property to metal oxide particles by boiling the particles in alcohols having not more than 8 carbon atoms (Japanese Patent Application Laid-open (KOKAI) No. 48-100398(1973)), a method of improving the dispersibility of hydrophilic pigment particles by adding an aliphatic alcohol and non-polar solvent to the particles, and then heating the mixture while stirring to fix the alcohol on surfaces of the particles (Japanese Patent Application Laid-open (KOKAI) No. 49-97822(1974)), a method of surface-treating metal oxide particles by chemically bonding ethylene glycol or the like on surfaces of the metal oxide particles and then reacting unreacted alcoholic hydroxyl group of the ethylene glycol with an ethylenically unsaturated bond-containing compound (Japanese Patent Publication (KOKOKU) No. 57-1483(1982)), a method of surface-treating metal oxide particles by reacting the monovalent alcohol-treated particles with a polyvalent alcohol to conduct an ester exchange reaction therebetween (Japanese Patent Application Laid-open (KOKAI) No. 4-92803(1992)), or the like.
Furthermore, there have been proposed inorganic pigment particles whose surfaces are coated with a surface active reaction product obtained by reacting a polyol having 3 to 4 hydroxyl groups and 3 to 7 carbon atoms such as trimethylol propane, with alkylene oxide in an amount of about 1 to about 5 moles per one hydroxyl group of the polyol (Japanese Patent Publication (KOKOKU) No. 53-18539(1978)), or the like.
However, any of these conventional pigment particles cannot exhibit a sufficient dispersibility when used in water-based paints.
That is, the above-mentioned ferromagnetic metal fine particles described in Japanese Patent Applications Laid-open (KOKAI) Nos. 5-225552(1993) and 5-225553(1993) whose surfaces are treated with the squaric acid or the aromatic organic acid having an anthraquinone skeleton (such as alizarin) cannot exhibit a sufficient negative charging property when used as pigment particles in water-based paints.
The above-mentioned pigment particles described in Japanese Patent Application Laid-open (KOKAI) No. 8-120191(1995) have an upper coating layer comprising polysiloxane. Since the polysiloxane is a relatively expensive treating agent and since it is necessary to conduct the coating treatment at a high temperature due to a high molecular weight thereof, the pigment particles are disadvantageous from industrial viewpoints.
The pigment particles described in Japanese Patent Applications Laid-open (KOKAI) Nos. 48-100398(1973), 49-97822(1974) and 4-92803(1992) and Japanese Patent Publication (KOKOKU) No. 57-1483(1982) are surface-treated mainly for imparting a lipophilic property thereto such that the particles can be used in organic solvent-type paints. Therefore, when these particles are used in water-based paints, sufficient dispersibility of the particles therein cannot be achieved.
Further, the inorganic particles described in Japanese Patent Publication (KOKOKU) No. 53-18539(1988) are coated with the surface active reaction product to facilitate the dispersion of the particles in both aqueous and organic solvents. However, as described in Comparative Examples hereinafter, the inorganic particles cannot show a sufficient dispersibility in water-based paints.
In consequence, it has been still strongly demanded to provide pigment particles which can show excellent dispersion facility, dispersion homogeneity and dispersion stability when used in not only organic solvent-type paints but also water-based paints.
As a result of the present inventors' earnest studies, it has been found that by mixing pigment particles with squaric acid in water or alcohol as a dispersing solvent, followed by filtering and drying, to form a lower coat comprising squaric acid on at least a part of surface of the pigment particle, and then mixing the squaric acid-coated particles with at least one kind of alkylene glycol represented by the general formula: C.sub.n H.sub.2n (OH).sub.2, wherein n is an integer of 2 to 10, to form an upper coat comprising alkylene glycol on at least a part of the said coated pigment particle, the obtained particles can show excellent dispersion facility, dispersion homogeneity and dispersion stability in paints, especially in water-based paints. The present invention has been attained on the basis of this finding.